Fair Value of Assets and Liabilities - Quantitative information about Level 3 instruments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Quantitative information about the significant unobservable inputs
Fixed income securities	$ 58,992	$ 57,839
Derivatives embedded in life and annuity contracts - Equity-indexed and forward starting options | Stochastic cash flow model
Quantitative information about the significant unobservable inputs
Weighted average projected option cost	1.74%	1.75%
Derivatives embedded in life and annuity contracts - Equity-indexed and forward starting options | Minimum | Stochastic cash flow model
Quantitative information about the significant unobservable inputs
Projected option cost	1.00%	1.00%
Derivatives embedded in life and annuity contracts - Equity-indexed and forward starting options | Maximum | Stochastic cash flow model
Quantitative information about the significant unobservable inputs
Projected option cost	2.20%	2.20%
Fixed income securities - non-binding broker quotes
Quantitative information about the significant unobservable inputs
Assets, fair value disclosure	$ 271	$ 307
Municipal - not rated by third party credit rating agencies
Quantitative information about the significant unobservable inputs
Assets, fair value disclosure	58	80
Significant unobservable inputs (Level 3)
Quantitative information about the significant unobservable inputs
Assets, fair value disclosure	840	762
Significant unobservable inputs (Level 3) | Derivatives embedded in life and annuity contracts - Equity-indexed and forward starting options | Stochastic cash flow model
Quantitative information about the significant unobservable inputs
Liabilities at fair value	(252)	(247)
Recurring | Significant unobservable inputs (Level 3)
Quantitative information about the significant unobservable inputs
Liabilities at fair value	(286)	(293)
Fixed income securities	606	558
Assets, fair value disclosure	$ 837	$ 738